A piezoelectric speaker has a merit that the piezoelectric speaker is easily thinned, since the piezoelectric speaker does not require a magnetic circuit, which is a drive system, unlike an electrodynamic speaker. However, the piezoelectric speaker has a demerit that when the same voltage is applied, the amplitude of a diaphragm is small and the reproduction sound pressure is low as compared to the electrodynamic speaker. In addition, many existing piezoelectric speakers are fixed at their peripheries, and thus are used for reproduction of middle and high frequencies and have difficulties in performing reproduction of low frequencies.
Therefore, in order to increase the reproduction sound pressure to enable also reproduction of low frequencies, a method of connecting a plurality of piezoelectric diaphragms in a vibration direction is conceived of (e.g., see Patent Literature 1). FIG. 13 is a conventional piezoelectric speaker described in Patent Literature 1. FIG. 14 schematically shows a vibration mode of the conventional piezoelectric speaker.
In FIG. 13, the conventional piezoelectric speaker includes an upper piezoelectric diaphragm 1, a lower piezoelectric diaphragm 2, connecting members 3, an edge 4, an upper frame 5, and a lower frame 6. The upper piezoelectric diaphragm 1 is a diaphragm with a bimorph structure including a substrate 1a and piezoelectric elements 1b and 1c. Similarly, the lower piezoelectric diaphragm 2 is also a diaphragm with a bimorph structure including a substrate 2a and piezoelectric elements 2b and 2c. 
End portions of the upper piezoelectric diaphragm 1 and the lower piezoelectric diaphragm 2 are connected to each other via the connecting members 3, and a central portion of the lower piezoelectric diaphragm 2 is adhered to the lower frame 6. In addition, the edge 4 is formed so as to cover tops of the upper piezoelectric diaphragm 1 and the upper frame 5. The edge 4 is provided to block sound emitted from the lower piezoelectric diaphragm 2 and the back side of the upper piezoelectric diaphragm 1. A stretchable laminate material is used for the edge 4. Furthermore, the polarities of the piezoelectric elements 1b, 1c, 2b, and 2c are set such that the upper piezoelectric diaphragm 1 and the lower piezoelectric diaphragm 2 are bended in opposite directions when a voltage is applied thereto.
In the conventional piezoelectric speaker, due to the above configuration, when a voltage is applied to the piezoelectric elements, the upper piezoelectric diaphragm 1 and the lower piezoelectric diaphragm 2 bend in directions opposite to each other. In other words, in the conventional piezoelectric speaker, a state where the upper piezoelectric diaphragm 1 and the lower piezoelectric diaphragm 2 become convex outwardly as shown in FIG. 14(a) and a state where the upper piezoelectric diaphragm 1 and the lower piezoelectric diaphragm 2 become convex inwardly as shown in FIG. 14(b) are alternately repeated. As a result, the amplitude of the upper piezoelectric diaphragm 1 is twice as large as that in the case of using a single piezoelectric diaphragm, and it is possible to increase the reproduction sound pressure. In addition, since the upper piezoelectric diaphragm 1 is supported by the edge 4 made of the laminate material, reproduction of low frequencies is enabled.